1. Field of the Invention
The present invention relates to an image processing method, an image processing apparatus, and an image forming apparatus comprising an image processing apparatus, and more particularly, to half-toning technology based on the effects of various errors occurring when dots are actually formed by a printer.
2. Description of the Related Art
An image forming apparatus, such as an inkjet printer, thermal printer, or LED printer, is known which forms images on a recording medium by means of dots. In an image forming apparatus of this kind, images are formed by means of a plurality of dots formed by liquid ink droplets, toner particles, or the like. Therefore, basically, an image is formed according to existence and nonexistence of dots on a recording medium such as white paper. Since the number of inks used is limited, a half-toning method is known which is used to achieve continuous tonal gradation by means of a limited number of inks.
For example, Japanese Patent Application Publication No. 6-245060 discloses a printing method and a system which are based on a model that generates an output color half-tone image corresponding to a color input signal.
This can improve the quality of the printed image by applying an error-diffusion halftoning technique, or a least-squares halftoning technique, to a color printer model in which the printed dots are approximately circular and are assumed to be larger than the minimum dimension required to cover the page completely.
Moreover, for example, Japanese Patent Application Publication No. 2002-240327 discloses a method in which the data of the pixels of the image to be recorded is quantized and ejection of ink from nozzles is controlled in accordance with this quantized data. In that method, when quantization error for each of the processed pixels, which is the error between the data achieved by quantization and the data originally to be recorded, is diffused (distributed) to the surrounding pixels, then the deterioration in image quality caused by the ink ejection state is reduced by correcting the quantization error on the basis of ejection state data of the nozzles corresponding to processed pixels.
Although Japanese Patent Application Publication No. 6-245060 discloses an error diffusion method using a plurality of color printer models, it involves a possibility in that it may be difficult to achieve sufficient improvement in image quality, because it is based on circular dots and the great effects on image quality caused by dot position error or size error are not taken into account.
Furthermore, in the method described in Japanese Patent Application Publication No. 2002-240327, the spreading of the ink is reflected in the actual value of the pixel under examination. For example, the ideal value is 255, but it may become a value of 310 or 180 due to variation. The differential with respect to the ideal value is reflected in the error, but this does not directly affect whether a dot is formed onto the position of the pixel under examination or not. Furthermore, although the effects of the spreading of ink from the peripheral pixels are reflected in the error of the pixel under examination, the error diffused from the adjacent pixels, which is taken into account when the pixel under examination is quantized, includes the effects of ink spreading from adjacent pixels to the adjacent pixels, and the quantization error in the adjacent pixels. However, the method described in Japanese Patent Application Publication No. 2002-240327 is not based on the effects of ink spreading from adjacent pixels into the pixels adjacent to the pixel under examination, and therefore, it is difficult that the effects of the projection of the ink droplets are reflected accurately in the pixel under examination.